The see of Ostia is conferred on the Dean of the College of Cardinals in addition to the see he already had. (Until 1914, the Dean received the see of Ostia and Velletri in place of the see he had earlier. The sees of Ostia and Velletri were separated in 1914.) The Dean used to be the longest-serving cardinal bishop, but is now chosen by the six cardinal bishops with the approval of the pope.

The increasing involvement of the Cardinal bishops in the administration of the papal curia resulted in a detachment from their dioceses. Therefore some of them, in particular the Bishops of Sabina and Velletri, have for centuries had auxiliary bishops and in 1910 Pope Pius X in his constitution Apostolicae Romanorum made this practice obligatory for all suburbicarian dioceses.[1] In 1962, Pope John XXIII in his decree Suburbicarii Sedis turned the Cardinal bishops into mere titular bishops and appointed diocesan bishops for Velletri-Segni, Porto-Santa-Rufina, Frascati, Palestrina, Albano and Sabina. The diocese of Ostia — the titular diocese of the Dean of the College of Cardinals — was merged with the diocese of Rome and was henceforth administered by the Roman vicar general. Though the diocesan bishops exercise all episcopal administrative functions, the Cardinal bishops still formally take possession of their titular dioceses.